The present invention relates to a feature or defect-location system including an in-line-inspection tool having an inertial navigation system (INS) and ties to a global positioning system (GPS). The present invention provides an accurate method of determining the location of pipeline features or defects detected by in-line-inspection tools such as magnetic flux leakage inspection tools (MFL) by comparing in-line-inspection data with inertial navigation system data and adjusting the inertial navigation system data to the accuracy of data collected from a global positioning system.
Pipelines must be monitored for changes in integrity, so that defects can be corrected before pipeline failure occurs. Various monitoring systems have been used to inspect internal pipeline walls and investigate the structural integrity of the pipeline walls.
While there are many different types of inspection tools, commonly known as "pigs," the most common types in use today may be classified generally as either geometry tools or metal loss tools.
Metal loss tools are often referred to as in-line inspection tools. In-line inspection tools are generally used to measure the pipeline wall thickness to detect any changes that may ultimately result in failure if left untended.
Geometry tools detect, measure and locate changes in the internal cross section of a pipeline. They are typically used prior to in-line inspection to ensure that the in-line-inspection instrument will pass safely through the pipeline and to collect data that helps in interpreting the information gathered by in-line inspection tools.
Two in-line inspection techniques are available for wall thickness measurement, namely magnetic flux leakage (MFL) and ultrasonic. Ultrasonic pigs require introduction of a liquid into a pipeline to couple the sensor signal to the pipe wall and are thus generally excluded from inspecting gas pipelines. The MFL method involves inducing a magnetic field into the pipe wall and sensing leakage of the field inside the pipe as the wall thickness changes. The MFL technique is the most commonly used technique to inspect large diameter gas transmission lines.
Once a severe or critical defect has been characterized, it must be located in the field so that the pipeline may be excavated and repaired. Most MFL, tools use some sort of above ground marker system to create a reference point along the pipeline. These markers are generally placed anywhere from one to two kilometers apart along the length of the pipeline. For example, timer boxes that include very accurate clocks may be placed above ground every one to two kilometers. When a pig passes under the ground beneath the timer box, sensors in the timer box sense the magnetic signal from the pig and the time the pig passes underneath the timer box is noted. The distance the pig travels between each marker is measured by the odometer wheels on the sides of the pig, and these measurements may be correlated with the features or defects identified by an in-line inspection tool and the time that the pig passes under each timer box. By determining how far the pig has traveled between the markers as measured by the odometer wheels to the identified feature or defect, the location of the feature or defect may be estimated.
Odometer measurements, however, may not be a very reliable method of determining the location of pipeline defects because of typical mechanical breakdowns inherent in most movable parts that may result in odometer wear and slippage, as well as horizontal chaining errors. The harsh operating environment within subterranean pipelines along with the long duration runs that MFL tools must endure tend to accelerate the wear and tear on the movable mechanical parts. In addition, the above ground markers are not always accurate and must be deployed during the inspection. Their deployment and maintenance are also very labor intensive.
There is a need for a more accurate, more reliable, and less labor intensive system for determining the location of features or defects in a pipeline.